Alkali metals are very reactive in their metallic or neutral state. Alkali metals are very reactive toward air and moisture and may catch fire spontaneously when exposed to these agents. To avoid the inherent hazards associated with their activity, the neutral metal must often be stored in vacuo or under an inert liquid such as oil in order to protect it from contact with the atmosphere, which may result in oxidation or other reactions. For example, sodium metal is often stored in Nujol oil which must, to avoid unwanted impurities, be removed prior to use in chemical reactions. This places severe restrictions on its shipment and use.
A number of compounds between alkali metals and silicon compounds have been prepared. For example, known compounds between sodium (Na) and silicon (Si) exist with stoichiometries that range from NaSi to Na(Si)6 (which is believed to be Na8Si46) to NaxSi136, with 1.5<x<24. (See Witte, J.; Schnering, H. G., “The Crystal Structure of NaSi and NaGe (in German)” Zeit Anorgan Allege Chemie 1964, 327, 260-273., Cros, C.; Pouchard, M.; Hagenmueller, P., “Two new Phases of the Silicon-Sodium System. (in French)” C. R. Acad. Sc. Paris 1965, 260, 4764-4767., and He, J.; Klug, D. D.; Uehara, K.; Preston, K. F.; Ratcliffe, C. I.; Tse, J. S., “NMR and X-ray Spectroscopy of Sodium-Silicon Clathrates” J. Phys. Chem. B 2001, 105.). The known compounds are formed by heating Na with Si to high temperatures, always at or above 500° C., and in some cases with removal of Na vapor by condensation on a cold surface. (See He, J.; Klug, D. D.; Uehara, K.; Preston, K. F.; Ratcliffe, C. I.; Tse, J. S., “NMR and X-ray Spectroscopy of Sodium-Silicon Clathrates” J. Phys. Chem. B 2001, 105, and Mayeri, D.; Phillips, B. L.; Augustine, M. P.; Kauzlarich, S. M., “NMR Study of the Synthesis of Alkyl-Terminated Silicon Nanoparticles from the Reaction of SiCl4 with the Zintl Salt, NaSi” Chem. Mater. 2001, 13, 765-770.). Mayeri et al. react silicon in the presence of sodium to a temperature of about 650° C. to form a sodium silicide. There have also been reports that a silicide of nominal composition NaSi2 can be prepared by heating Na with quartz (SiO2), although the evidence for this composition is slim. (See Novotny, H.; Scheil, E., “A Ternary Compound in the System Aluminum-Silicon-Sodium (in German)” Metallforsch. 1947, 2, 76-80.).
It has often been assumed that NaSi is so reactive that it must be considered to be pyrophoric, or able to spontaneously ignite in the presence of air. It has also been recently characterized as “air and moisture sensitive.” (See He, J.; Klug, D. D.; Uehara, K.; Preston, K. F.; Ratcliffe, C. I.; Tse, J. S., “NMR and X-ray Spectroscopy of Sodium-Silicon Clathrates” J. Phys. Chem. B 2001, 105.). This study showed, however, that the clathrate structure of Na8Si46 is non-reactive toward air and moisture. Id. However, it is generally agreed that the reaction of NaSi with water is rapid and “violent”, such that the heat of reaction can ignite the hydrogen formed, just as occurs in the reaction of alkali metals with water. This places severe restrictions on storing and handling NaSi without keeping it in vacuo or under an inert atmosphere to avoid its inherent hazards.
A major problem with the synthesis of NaSi materials has been the need to heat Na and Si in a closed system to prevent the condensation of Na at cold sites. Conventionally, for example, to prepare polycrystalline NaSi powder, excess Na was heated with Si in a molybdenum (Mo) tube that was welded shut and heated for three days at 500° C. (See Mayeri, D.; Phillips, B. L.; Augustine, M. P.; Kauzlarich, S. M., “NMR Study of the Synthesis of Alkyl-Terminated Silicon Nanoparticles from the Reaction of SiCl4 with the Zintl Salt, NaSi” Chem. Mater. 2001, 13, 765-770.). In another study, a stainless steel container was used. (See He, J.; Klug, D. D.; Uehara, K.; Preston, K. F.; Ratcliffe, C. I.; Tse, J. S., “NMR and X-ray Spectroscopy of Sodium-Silicon Clathrates” J. Phys. Chem. B 2001, 105.).
A need exists, therefore, to prepare an alkali metal silicide composition conveniently and inexpensively, so that it may be easily handled in air without a significant loss in its ability to reduce water. This reduction reaction would be able to produce large amounts of hydrogen per unit mass of the solid.